Nucleic acid hybridisation

• Heating DNA to about 100 °C breaks the hydrogen bonds between bases, and the double helix separates into single strands (denaturation). On cooling, single strands tend to re-anneal to form a double helix. Any two single strands of DNA or RNA will form a double strand if they have at least short stretches of complementary bases.
• The degree of annealing is measured by the heat needed to separate the strands again: the more complementary the strands, the tighter they bind and the more heat is needed.
• Mixing nucleic acids from two sources, heating and cooling, gives hybrid double helices with one strand from each source.

Uses of hybridisation:

1. To tell whether a particular gene is present in a genome and in what amount. A single-stranded radioactively labelled probe complementary to the gene is mixed with the sample; the rate at which labelled double helices form shows the gene's concentration.
2. To judge evolutionary relationships: the more closely related two species, the more similar their DNA and the more hybridisation between them.